The gas turbines that power electrical generators discharge exhaust gases at extremely high temperatures. Heat recovery steam generators (HRSGs) extract the heat from the gases to produce steam that powers steam turbines that in turn drive more electrical generators.
The typical HRSG includes multiple sections located one after the other in the flow of hot exhaust gases from a gas turbine. Among these sections are an economizer for elevating the temperature of feed water, an evaporator for converting high temperature water into saturated steam, and a superheater for converting the saturated steam into superheated steam. Many HRSGs have more than one economizer, evaporator, and superheater. Some are equipped with reheaters. The evaporator may be a circulation-type evaporator that has an overhead steam drum from which the steam produced by the evaporator is directed to the superheater. Water circulates through the steam drum as well. On the other hand, the evaporator may be a once-through evaporator that converts water into saturated steam without the steam passing through a steam drum. Water and impurities also collect in the lower regions of the superheater, reheater, and economizer. Unless the water is purged from time to time from the sections of an HRSG, minerals and other impurities may become so concentrated in those sections that they foul the sections or cause foaming that reduces heat transfer. Hence, the sections of an HRSG are provided in their lower regions with valves that, when opened, release water and as to some sections steam as well. Generally, with the exception of overhead steam drums for circulation-type evaporators, the valves are located near the bottom of the HRSG, and that is near grade. When the valves are opened they release drain water.
Steam drums, when present, represent the greatest source of discharged water. The typical steam drum operates at the pressure of the evaporator of which it is a part, and that pressure is considerable. The discharge is continuous in the form of blowdown and at a high velocity. In addition, the typical steam drum has a blowoff port and valve at its bottom for providing intermittent discharges in the form of blowoff that may contain solids.
Regulations governing the use of sewer systems limit the temperature of water that may be discharged into such systems—and the temperature of blowoffs and blowdowns from circulation-type evaporators and drain water from other sections of an HRSG often exceed the high temperature limit. As a consequence, HRSGs are equipped with blowoff tanks where the blowoff and blowdown and also drain water are mixed with cooler water to reduce the temperature of the mixture low enough to comply with sewer regulations.
Other types of boilers experience similar problems. Unless the water in the lower regions of such boilers is purged from time to time through blowoffs, minerals that are initially dissolved in the water become concentrated to the extent that they precipitate as solids. Hence, more traditional boilers may likewise be coupled with blowoff tanks.
The typical blowoff tank A (FIG. 1) that is currently utilized to receive blowoff, blowdown, and drain water from HRSGs, and other boilers as well, includes a vessel 2 of generally cylindrical configuration that is oriented vertically with its lower end only slightly above grade. The tank A has inlets 4 that pass through the side wall of the vessel 2, generally tangentially, at the upper regions of the vessel 2 where the vessel 2 has a liner or wear plate 6 formed from stainless steel. The blowoff and blowdown lines and drain water lines from the various sections of an HRSG are connected to the inlets 4. The tank A also has a primary drain line 8 that originates near the very bottom of the vessel 2, rises within the vessel 2 through its lower region, and emerges from the vessel 2 slightly below the midpoint of the vessel 2. At the very bottom of the vessel 2 a secondary drain line 12 emerges from the vessel 2, but it is normally closed by a valve 14. Both drain lines 8 and 12 lead to a sewer. At its upper end the vessel 2 is fitted with a vent 16 that discharges into the atmosphere.
Blowoff, blowdown, and drain water pass through the blowoff, blowdown, and drain lines and discharge into the vessel 2 at the inlets 4. Here it enters the vessel 2 tangentially along the wear plate 6 and produces a vortex that allows steam that flashes off to escape through the vent 16. The water collects in the vessel 2 and is maintained at an elevation defined as the normal water level. Since at least some of the lines from the HRSG leave the various sections that they purge only slightly above grade and then must rise to the level of the inlets 4 to discharge water into the vessel 2, water becomes trapped in those lines. This may produce water hammer when the valves that purge the sections are opened. Moreover, the trapped water, will corrode the lines. The inlets 4 must accommodate flowing water that is high in temperature and erosive, and thus the inlets are complicated and costly. Moreover, the tangential entry of the inlets 4 into the vessel 2 makes the construction even more complex. When the tank A has multiple inlets 4, it must have increased length and the same holds true for the liner 6 as well.
To avoid the problems created by water trapped in the blowoff, blowdown, and drain lines, sometimes operators of HRSGs will install blowoff tanks A below grade in pits. In that way the water drains from the lines. But pits add expense and make servicing the blowoff tanks difficult.